minigbm: For BO_USE_SW usage buffer need not be linear

It is incorrect to force linear for BO_USE_SW usage as mapImage
in dri_bo_map converts tiled data and returns linear.

BUG=b:78200321
TEST=drm_cursor_test completes within 20 seconds.

Change-Id: Ifa3ca2218cee9ff52b049a984a70a54f4899965d
Signed-off-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-on: https://chromium-review.googlesource.com/1041448


Commit-Ready: Drew Davenport <ddavenport@chromium.org>
Tested-by: Deepak Sharma <deepak.sharma@amd.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>